1.
the first train of the day
the earliest train service on a particular route or line
Forms
Usually shortened to 始発, which also means the first departure of a bus or the station a line starts from (始発駅).
Usage
First trains run from about 4:30 to 5:30 a.m., depending on the line. 始発で帰る (to go home on the first train) implies having stayed out all night after missing the last train.